Yeah. We should get back to insisting whether or not Nobles are monsters. That's a key point.
Nobles are monsters if you are intended to fight them. Nobles are NPC's if you're intended to interact with them non-violently. A Noble can be both, or switch from one to the other depending on circumstances.
I just find the MM quote to be hilarious because the official definition of a monster is so all-encompassing it could actually mean the PC's are monsters (and I suppose, to the monsters, they are!).
The reality is simple- if you want your Noble to have combat stats, you use a stat block. In every edition of the game, you'd do that. But only rarely as a "Noble". Until the Aristocrat class was created, most nobles you'd find statted up in D&D had class levels, like the King of Cormyr in the Forgotten Realms being a 20th level Cavalier (?!). And often, these stats were to keep players from just assassinating leaders of state, lol.
It wasn't until 4e, and now 5e, that you'd have a generic "Noble" monster.
So moving away from that, there's now the issue of what being a noble
means in D&D. Is a Barbarian Chieftain, the son of the old Chief, and the son of the Chief before him a noble? Would his daughter be a Barbarian princess and thus a noble, despite the fact that she can lop a man's head off with an axe?
Is a Frost Giant Jarl a noble? An Archdevil? How about the wealthy mayor of a town?
Is being a noble in a setting the result of the Divine Right of Kings? Do you have to be 2/3 God and one part Man? Or are you just a worshiped as a God?
Can you buy your way into the aristocracy or be granted a Knighthood? Is Conan a noble because he sat down on a throne and declared himself King?
There's no "one size fits all" description here, which does give a lot of weight to the argument that the Noble Background might not work when you interact with a culture you're not a part of. Is the Daimyo of House Kakita going to accept that this "round-eyed white demon" is actually a Noble from a distant land? Would it matter if he did?
The Background Features are written for a game where there is some mystical quality that sets a noble apart- it's a staple of Western European folklore. In The Matter of England or the Matter of France, there is something special about nobles, and most people recognize that on sight.
What's interesting is that not all D&D worlds are like that. Being a King isn't a big deal in the Forgotten Realms- having an army is! Just ask the Tuigan. But walk into the Kryptwood and Old Gnawbone will devour you the same as he would anyone else!
Being a King isn't particularly special in most D&D settings (Birthright being a notable exception) in of itself. So yeah, the background does kind of stick out like a sore thumb- it works if that's the kind of game you're running. But the feature (and the DMG) are written in a way that ways "this is how this works in D&D" as a general statement. Very strange.
Now me personally, I don't mind the players having special destinies and people noticing that there's more to that "moisture farmer" or "assistant pig-keeper" than meets the eye. The concept of being a
Ta'veren is fine with me (I'd draw the line at a
Kwisatz Haderach though)...to a point.
And that point is, when one player's story runs the risk of taking too much spotlight, or interfering with another's. As Matt Colville once quoted about something a friend said, "I didn't realize D&D is a game where your fun could be ruined by someone else's fun".
But all that having been said, if you don't want PC's who are inherently special, and have to earn their legend from scratch, something like the Noble Background isn't going to work out as written.
Whoever came up with the Backgrounds Concept was coming from a place where WotC was willing to define what D&D
is. Which as we know, they quickly backed away from.
D&D is just a system of rules, man. The game is what you make of it.